This invention is about coils used in magnetoencephalographic experiments to determine the position and orientation of the magnetometer, and a means of attaching the said coils to the head. In magnetoencephalography (MEG), the weak magnetic fields elicited by the human brain are measured. The method is gaining gradually a more important role in medical research and diagnostics. In particular, it is possible to investigate the brain functions and disorders in a human being without touching the person or exposing him to electromagnetic radiation or radioactive tracers. In contrast to the widely used electroencephalogram (EEG), in which the electric potential distribution is measured on the surface of the scalp, the magnetoencephalogram suffers far less from distortions caused by inhomogeneities in the conductivity of the human tissue. Therefore, it is possible to locate source currents related to brain activity with a spatial and temporal resolution of a few millimeters and milliseconds. The method has been reviewed in detail, for example, in CRC Critical Reviews in Biomedical Engineering, volume 14 (1986), issue 2, pp. 93-126.
Instruments used in MEG should be able to detect magnetic signals whose magnetic flux density is typically 100 fT or less. In addition, the measurement is to be performed simultaneously at several locations; the measurement of even more than one hundred magnetic signals from all over the head is necessary. The only sensor capable of detecting these minute signals is the so-called Superconducting Quantum Interference Device (SQUID) magnetometer. The operation of the device has been explained in detail in an article in Journal of Low Temperature Physics, volume 76 (1989), issue 5/6, pp. 287-386.
In order to locate accurately the current distribution caused by the brain's activity from the measured magnetic field distribution, the position and orientation of the magnetometers with respect to the head has to be known. Mechanical means of determining the position and orientation are too elaborate and prone to errors; therefore an automatic position indicator system is needed.
In magnetoencephalographic experiments, one typically uses a locating method employing at least three transmitter coils which are attached to the subject's head; the positions and orientations of the coils are accurately determined prior to the measurement by some other means. The system also includes a current supply, a magnetic field detector, and a computer to determine the coordinates of the coils on the basis of the measured magnetic field. The system is capable of automatically determining the position and orientation of the magnetometer during the measurement. As the magnetic detector of the position indicator system, a multichannel SQUID-magnetometer is particularly useful (cf. Review of Scientific Instruments, volume 58 (1987), issue 11, pp. 2145-2156 or Advances in Biomagnetism, edited by S. J. Williamson, M. Hoke, G. Stroink, and M. Kotani, Plenum Press, New York 1989, pp. 689-692 and pp. 693-696). Another known position indicator system U.S. Pat. No. 4,793,355 employs orthogonal coil sets comprising three coils as transmitters and receivers; the method is based on the induction principle. The receiver coils are attached to the head of the subject and the transmitter coils to the magnetometer.
It is crucial for the accuracy of the position indication that the coils stay firmly on their positions during the measurement; it is also important to have them as close to the measurement area as possible. Usually, the coils are wound, by hand or by other means, from wire around a non-magnetic core, as in the references cited above. Three such coils are cast in epoxy resin, forming a coil set. The coil sets are attached to the subject's head by means of a flexible headband or with adhesive tape.
The main drawback of the prior-art technique is that the coil sets are too big to be attached to the actual measurement area, i.e. between the head and the SQUID magnetometer. On the other hand, the present techniques do not provide a possibility to attach the coils firmly at an arbitrary position on the cortex. When using a headband, the area where the attachment of the coils is possible is limited, and the hair prevents a firm adhering of the coils with tape. The accuracy of dimensions of wire-wound multi-turn coils is not very good, and taking into account the stray fields or eliminating them is very elaborate. The determination of the effective dipole moment of the coil is therefore difficult; the error reflects itself correspondingly in the results of the sensor position determination.
In another context, several possible means of attaching the coils to the head have been described. For example, DE-2 206 913 deals with a stiff headband to be placed around the subject's head for attaching EEG electrodes. The headband is hinged on the back, and it is tightened around the head using a punched rubber strap or, alternatively, a spring-loaded hinge. The electrodes are attached to the headband via stiff connector wires. The wires are bent in a spring-like manner to press the electrodes against the head.
The solution presented in the said reference has several drawbacks. Due to the completely stiff structure, the construction cannot be made flat enough not to considerably hamper the ease of operation of the magnetometer. As a result, the multichannel magnetometer must be placed rather far from the head, resulting in a substantially deteriorated signal-to-noise ratio. In addition, the means of attaching the electrodes (FIG. 1 of the reference cited above) is not suitable for position indicator coils because it utilizes only one support arm: a well-defined position and orientation (along the tangent plane) on the head, accurate enough for the MEG method and stationary during the measurement, cannot be guaranteed; coils may move easily during the measurement, for instance, when moving the magnetometer. A third drawback is that the electrodes can only be attached to certain predetermined positions.
DE-2 124 704 describes an elastic, hollow headband used to attach electrodes. The electrodes are located on the inside surface of the headband, and when the headband is pressurized, e.g., by compressed air the electrodes are pressed against the head of the subject. This is not a solution either, because the construction cannot be made flat enough not to hamper the placing the magnetometer as close to the head as possible. In addition, the electrodes can be attached only to predetermined places.
In EP-A2-399 499 magnetometer elements are attached to a stiff shell (cf. FIG. 6 of the reference). The method is not suitable for position indicator coils, since the resulting structure is completely stiff. The coils cannot be attached freely anywhere on the cortex on every subject so that the distance from the magnetometers to the head would not substantially increase. A solution to this problem would be an individual, tight-fitting shell for each subject, but this would certainly considerably annoy the subject. In addition, making individual shells would make clinical measurements with patients more elaborate and increase the time consumption and cost for preparing the measurement excessively.
EP-A3-199 214 mentions two means of attaching electrodes to the subject's head. The first one employs an elastic cap which can be set very tightly against the head. The second method includes a stiff headband which can be tightened around the head. A stiff strip is then attached to this headband in a fixed and stiff manner; this strip goes over the head. Neither of the two methods is suitable for position indicator coils, since the elastic cap does not guarantee that the coils are not moved during the measurement and the stiff construction does not allow a free selection of measurement points on the cortex. The stiff construction is also uncomfortable for the subject.
The use of planar coils, fabricated using thin film or thick film techniques and encapsulating them inside epoxy resin as well as the use of parallel conductors patterned on flexible, insulating substrates is known as such, for example, from DE-A1-3 135 962, DE-C3-2 315 498, U.S. Pat. No. 4,749,946 Transactions on Magnetics, vol. MAG-17, issue 1 (January 1981), pp. 400-401 and DE-A1-3 148 192. None of the features described in the above references solves alone the problem how to make a small, dimensionally accurate coil the stray fields of which are negligible, including current supply leads, and which is, in addition, as flat as possible and can be reliably and easily attached to the head.